(1) Field of the Invention
The present invention generally relates to a dynamic behavior estimate system of an automotive vehicle, and more particularly to a dynamic behavior estimate system which discriminates an instability of an automotive vehicle when operating in a curved path, in accordance with one of vehicle behavior parameters, including a yaw rate, a lateral acceleration and a steering angle.
(2) Description of the Related Art
A conventional dynamic behavior estimate system discriminates an instability of an automotive vehicle when operating in a curved path, based on a relationship between a slip angle of wheels of the vehicle and a cornering force acting on the wheels, which is disclosed in, for example, U.S. Pat. No. 4,809,183.
Generally, the wheels of the vehicle have a proper road holding ability when the slip angle and the cornering force have a linear relation. The slip angle of a wheel is an angle of a central plane of the wheel to a forward running direction of the vehicle.
The conventional dynamic behavior estimate system, disclosed in the above publication, carries out a discrimination of the instability of the vehicle by detecting whether the slip angle and the cornering force have a linear relation with respect to each of the front wheels and the rear wheels of the vehicle.
More specifically, if the slip angle and the cornering force for both the front wheels and the rear wheels are detected to have a linear relation, the above dynamic behavior estimate system determines that no instability of the vehicle occurs or the vehicle operates in a stable condition. If the slip angle and the cornering force for the rear wheels are detected to have a non-linear relation, the above dynamic behavior estimate system determines that an instability of the vehicle, or "spin" of the vehicle, occurs. If the slip angle and the cornering force for the front wheels are detected to have a non-linear relation, the above dynamic behavior estimate system determines that an instability of the vehicle, or "drift-out" of the vehicle, occurs.
However, in order to detect the slip angle of the wheels, the above dynamic behavior estimate system is required to sense an absolute speed of the vehicle in the front-to-rear direction and an absolute speed of the vehicle in the left-to-right direction. Therefore, the above dynamic behavior estimate system must use an absolute vehicle speed sensing unit which is capable of sensing such absolute vehicle speeds, and the absolute vehicle speed sensing unit is expensive.
Accordingly, there is a problem that it is difficult for the above dynamic behavior estimate system to realize a discrimination of the instability of the vehicle with a low cost.
In addition, another method of detecting the slip angle of the wheels by using various vehicle behavior parameters including a vehicle speed, a lateral acceleration and a yaw rate, is known. In order to realize such a method, it is necessary that a dynamic behavior estimate system includes several sensors which sense the several vehicle parameters including at least the vehicle speed, the lateral acceleration and the yaw rate.
Further, in the above method of detecting the slip angle, calculations used to discriminate the instability of the vehicle are performed in accordance with detection signals output from the plurality of the sensors. The results of the calculations inevitably include errors of the detection signals from the sensors which may be accumulated. Therefore, the accuracy of the discrimination by the above method is likely to become worse, and it is difficult to realize an accurate discrimination of the instability of the vehicle.